1. Field of the Invention
The invention generally concerns radar transmitter pulse modulators and, more specifically, relates to such modulators employed with travelling wave tube radio frequency power amplifiers.
2. Description of the Prior Art
Quite probably the classical solution for this problem is a pulse-top clipper which can be arranged to eliminate the pulse-top ripple by substantially clamping the instantaneous power pulse amplitude at a predetermined level. Although such a circuit is relatively simple, there are some serious disadvantages associated therewith. One disadvantage is the necessity of dumping pulse energy in excess of the clipping level during the interpulse period. In addition to the significant waste of energy thereby resulting, the energy dumping process inherently makes the clip level a function of interpulse period (i.e., of pulse repetition frequency). Moreover, tilt of the pulse-top results causing the same type of intra-pulse phase and/or amplitude variations of the radio frequency power at the travelling wave tube output as are caused by the pulse top ripple. Another possible approach to the problem of reducing the pulse-top ripple which usually suggests itself to those of skill in this art in similar situations is the application of feedback or feed-forward, i.e., by putting a closed loop around the entire device. Unfortunately, such a loop would be complex, would necessarily be required to handle a high level of power and must be capable of rapid (high bandwidth) operation. Still further, such a closed loop approach must discriminate between rise and fall times of the power pulse and the ripple, preserving the former and cancelling the latter.
Still further, the power modulation pulse to the travelling wave tube can be generated as a high powered gate without the use of resonant or artificial transmission line elements present in the so-called line type modulators. Generation of such a high powered gate necessarily requires enormous high voltage and high current power supply backup. Accordingly, that alternative cannot be seriously considered.
The so-called line type pulse modulator is a fairly standard and well-known device for the generation of the power pulse required to operate pulsed radar transmitting generators such as magnetrons, amplitrons and travelling wave tubes.
The selection of a travelling wave tube for a power amplifier application in a modern radar system is based on its known advantages as applicable to overall system characteristics which are not, per se, a part of the present invention. The criteria for selection of a travelling wave tube are well known, however, and given that selection, the choice of a line type modulator follows.
The line type modulator is basically a section of artificial delay line including passive inductive capacitive and resistive circuit elements, the characteristics of which are relatively stable and repeatable at least over short and intermediate times.
It is known that pulse-operable, power, travelling tubes are inherently amplitude and phase modulated by variations in modulation pulse amplitude. Since the line type modulator is simple and relatively inexpensive for use where single or at least non-agile pulse width is involved, its use is highly desirable.
Unfortunately, however, realizable (nominally twenty section plus or minus) pulse forming networks produce significant pulse-top ripple (usually on the order of one percent). Where modern radar signal processing (such as for MTI cancellation) is to be used, this ripple must be reduced in the travelling wave tube system, particularly where phase coded transmitted pulses are employed for pulse compression upon reception. The previously mentioned phase and amplitude perturbations produced by the modulation pulse-top ripple in a travelling wave tube system tends to deteriorate the performance of such phase-coded systems. In general, a maximum ripple of only 0.01 percent can be tolerated if the received signal correlation time side lobes are to be held to not more than 40 db.
The classical and known prior art approaches which might be expected to provide reduction of the modulation pulse-top ripple are not satisfactory for the reasons given.
In accordance with the foregoing, it can be seen that for modern radar systems having the characteristics and capabilities referred to, there is an urgent need for apparatus capable of effectively yet inexpensively affording the desired degree of pulse-top ripple reduction or compensation for use in the travelling wave tube system aforementioned. The manner in which the present invention deals with the prior art disadvantages to produce such a simple and effective pulse-top ripple compensator for the travelling wave tube application as well as for other potential applications will be understood as this description proceeds.
The invention makes use of digitizing and digital encoding techniques for constructing an image wave representative of the pulse-top ripple. This image is stored as a series of digital sample values taken throughout the time duration of the ripple waveform and continuously updated by comparison of the actual ripple with the digitally stored image converted back to analog values while the ripple waveform xe2x80x9cridesxe2x80x9d on the top of a large voltage pulse.
Forty-six thousand volts peak amplitude modulation pulses are used in one application in which the invention may be used. However, the compensating stored and re-converted ripple signal is applied at a relatively low potential with respect to ground to the travelling wave tube collector electrode. It will be noted that the particular type of travelling wave tube illustrated in the drawings does not have a grid electrode, but rather is intended for negative going pulse application to the cathode element for pulse operation.
The application of the compensating waveform is of the same polarity when applied to the collector electrode as is extant at the travelling wave tube cathode, the result being a substantially constant (save only a small residual ripple component) potential across the travelling wave tube, i.e., from cathode to collector. Accordingly, the amplitude and phase perturbations introduced by the travelling wave tube are greatly reduced. This effect constitutes fulfillment of the general objective of the invention.
The details of a representative implementation of the invention are presented hereinafter.